As an air flow rate apparatus for measuring an intake air flow rate arranged in an electronically controlled fuel injection unit of an internal combustion engine in a vehicle or the like, air flow rate measuring apparatuses of thermal type have been widely used because of capability of directly detecting a mass air flow rate. Therein, its heating resistor is constructed by winding a platinum wire around a bobbin and then coating the wire with glass, or by forming a thin film resistor on a ceramic substrate or a silicon substrate.
As the methods of detecting a flow rate, there are a type in which a heating resistor is heated up to a given temperature and a flow rate is directly detected from a value of current flowing through the resistor when a fluid flows, and a type in which temperature detecting resistors are arranged in both sides of a heating resistor and a flow rate is detected from a temperature difference between the temperature detecting resistors.
Particularly in vehicles, in a case of pulsating flow having a large pulsating amplitude of an intake air flow rate and partial reversed flow, which may take place under a low rotation speed and a heavy load condition in an engine having four or less cylinders, the conventional air flow rate measuring apparatus requires an output signal corresponding to the air flow direction because the accuracy of the measured flow rate becomes poor. The type, in which temperature detecting resistors are arranged in both sides of the heating resistor and an air flow rate is detected from a temperature difference between the temperature detecting resistors, is suitable for detecting an output signal under the condition of existing of the reversed air flow because the output signal corresponds to the flow direction.
Since each of the above two types has advantages and disadvantages depending on the use, a type combining the above both types using an analog circuit is disclosed in Japanese Patent Application Laid-Open No. 9-318412, and in Japanese Patent Application Laid-Open No. 11-51954. That is, since the temperature difference output signal having a comparatively high sensitivity is deteriorated in a high flow rate side due to saturation of the sensitivity, the temperature difference output signal is output by adding the output signal of the direct detecting type having low sensitivity in the low flow rate side and high sensitivity in the high flow rate side using a differential amplifier.
As methods of compensating the output signal of temperature difference between the temperature detecting resistors having the comparatively good sensitivity other than the above described method of compensating the sensitivity, a method of compensating the output signal by dividing by a temperature rise of a heater is disclosed in Japanese Patent Publication No. 6-63801, and a method of compensating temperature is disclosed in Japanese Patent Publication No. 6-64080.
In addition, as another method of compensating the output signal of temperature difference between the temperature detecting resistors particularly for vehicles, a method of compensating the output signal by detecting temperature of a medium is disclosed in Japanese Patent Application Laid-Open No. 6-160142.
On the other hand, as digitized methods using an A/D converter, a method of compensating a zero point depending on the output signal of a temperature detecting resistor is disclosed in Japanese Patent Application Laid-Open No. 6-230021. Further, a method of digitally compensating temperature is disclosed in Japanese Patent Application Laid-Open No. 11-94620.
In the prior art described above, particularly, as the methods of compensating the accuracy of output signal of the temperature difference between the temperature detecting resistors, various types mainly using an analog circuit have been proposed. In the analog circuit, various kinds of devices different in the measuring range of flow rate corresponding to the uses are required, and the circuits and the adjustments become complex in order to improve the accuracy. Therefore, these increase the cost. Although the type of digitally compensating the zero point or the type of digitally compensating temperature has been studied, adjusting of sensitivity of the whole sensors has not been considered.